PYROLYSIS AND COMBUSTION OF A METAL RICH SOLID REFINERY RESIDUE

This study addresses pyrolysis and combustion of a heavy hydrocarbon residue of the refinery industry. The material has good calorific value (HHV= 34050 kJ/kg), is solid at room temperature and has a high content of metals (Mo, V) and impurities. The effect of these metals on pyrolysis and combustion has been investigated. The activity included first of all the physical chemical characterization of the raw sample before and of samples of char and ash by means of several techniques: ICP, SEM EDAX, XRD. Moreover pyrolysis and combustion have been investigated by TGA and in small scale reactors of the fixed and fluidized bed type. Metals are relatively stable upon pyrolysis (with possible exceptions of sodium and lead), but sensitive to combustion. In particular, Mo, very abundant in the raw sample, is only marginally retained in the ash after combustion, because of the large volatility of this metal in the oxidized state. TGA experiments show interesting effects of the metal content on the pyrolysis and char combustion behavior: when the sample is heated up under mildly oxidizing atmospheres (i.e oxygen concentration of <1%), the metals promote the uptake of oxygen at temperatures below 400°C in parallel with char formation and affects also the graphitization of the carbon structure The experiments in the tubular reactor show that the composition of the pyrolysis gas is strongly affected by the presence of even low concentration of oxygen. Despite the fact that a catalytic action of the metals could be expected, experiments in a fluidized bed reactor show that times of both pyrolysis and char combustion are larger than for most conventional and non conventional solid fuels.